Image display apparatus

ABSTRACT

A surface light source device has a light conducting plate with a light source disposed adjacent to one of its side surfaces. At least one pattern is formed on the upper light emitting surface of the light conducting plate and/or the lower surface. The pattern is provided such that the sum of average slope angles of the light emitting surface and the opposite surface on a first sectional surface which is perpendicular to both the light incident surface and the light emitting surface is greater than the sum of average slope angles of the light emitting surface and the opposite surface on a second sectional surface which is parallel to the light incident side surface. Image display apparatus incorporating such a surface light source device have improved brightness and other characteristics.

BACKGROUND OF THE INVENTION

[0001] This invention relates firstly to surface light source devicesand in particular to such devices of the edge light type. This inventionrelates additionally to optical elements such as prism sheets which areused in such surface light source devices, as well as to apparatus usingsuch surface light source devices such as image display apparatus,automatic teller machines and game tables.

[0002] Liquid crystal (LC) display apparatus, because they have thefavorable characteristics of being light and thin, have been used asdisplay devices not only for lap-top and book-type personal computersand word processors but also for electronic notebooks, portabletelephones, LC television sets, various portable terminals and videocameras. More recently, they are also being used as display apparatusfor measurement instruments such as time counters, overhead display ofvirtual reality and LC projectors.

[0003] Among these LC display apparatus, there are those having avertically downward-facing surface light source device disposed on theback surface of a LC display panel (hereinafter referred to as the LCDpanel), as well as those having an edge-light type surface light sourcedevice. FIGS. 1A and 1B show a surface light source device 1 of theformer kind, having a linear light source 4 such as a cold cathode raytube (a fluorescent tube) disposed on the back surface of diffusionplates 2 and 3 and a reflector 5 further behind the linear light source4 such that the emitted light from the linear light source 4 can bediffused by the diffusion plates 2 and 3 and uniformly projected outfrom the projecting surface. Because a plurality of linear light sourcescan be disposed behind the diffusion plates, an LC display apparatususing such a vertically downward-facing surface light source device canprovide a high degree of brightness. For obtaining a uniform brightnessover the entire light-emitting surface, however, a certain distance mustbe maintained between the light source and the diffusion plates, causingthe overall thickness of the surface light source device to increase.This makes it difficult to produce thin LC display apparatus.

[0004] Edge-light type surface light source devices have the advantagethat the light source can be made thin because the linear light sourceis positioned at a side of a light conducting plate. Because of thisadvantage, more and more apparatus are coming to use edge-light typesurface light source devices, as the demand to reduce the thickness ofLC display apparatus is becoming greater.

[0005]FIG. 2 shows an edge-light type surface light source device 6,with a portion removed, including optical elements such as a linearlight source 7, a reflector 8, a light conducting plate 9, alight-reflecting plate 10, a diffusion plate 11 and a pair of converginglens plates 12 and 13. The linear light source 7 and the reflector 8 aredisposed by a (light-incident) side surface of the optically transparentlight conducting plate 9 such that the light emitted from the linearlight source 7 enters the light conducting plate 9 through this sidesurface either directly or after being reflected by the reflector 8.Side-surface reflecting plates (shown at 14 in FIG. 5) of a metallicdielectric material with a rough surface are provided on side surfacesof the light conducting plate 9 other than the light-incident surface. Acold cathode ray tube (fluorescent tube) is shown as the linear lightsource 7. A straight single tube or an L-shaped tube may be used,depending on the brightness of display required of the LC displayapparatus 6.

[0006] A diffusion layer 15 is formed on the lower surface of the lightconducting plate 9, and the light-reflecting plate 10 is disposedtherebelow. The diffusion layer 15 may be produced by depositing dots oflight-diffusing paint or the like by a screen-printing method such thatthe area of the diffusion layer 15 increases gradually as the distancefrom the linear light source 7 increases, as shown by examples in FIGS.3A and 3B. Alternatively, the diffusion layer 15 may take the form, asshown in FIGS. 4A and 4B, of indentations (or protrusions) provided onthe lower surface of the light conducting plate 9. In this case, thediffusion layer 15 becomes wider as the distance from the linear lightsource 7 increases. The light which is passing through the lightconducting plate 9 is diffused by the diffusion layer 15 either after itis totally reflected at the upper surface of the light conducting plate9, simply reflected by the light-reflecting plate 10 or at the uppersurface of the light conducting plate 9 or directly by entering thediffusion layer 15. Only that small portion of the light which did notundergo total reflection at the top surface escapes. Since the area ofthe diffusion layer 15 increases as the distance from the linear lightsource 7 increases, diffused light is emitted out of the lightconducting plate 9 at a uniform brightness over the whole of the lightconducting plate 9.

[0007] The diffusion plate 11 and the pair of converging lens plates 12and 13 are stacked on the upper surface of the light conducting plate 9.The diffusion plate 11 comprises a synthetic resin sheet or film withits surface processed to provide fine roughness. The portion of lightwhich escaped through the upper surface of the light conducting plate 9is diffused by the diffusion plate 11. The converging plates 12 and 13each have a parallel array of sectionally triangular prisms such thatthe arrays on the upper lens plate 13 and the lower lens plate 12 areperpendicular to each other. Thus, the light which has been diffused bythe diffusion plate 11 is focused by the lens plates 12 and 13 in twodirections and emitted out of the light conducting plate 9 nearlyperpendicularly to its upper surface.

[0008] The efficiency, by which light from the linear light source 7 canbe led to the upper surface, will be discussed next. Assume now that thediffusion layer 15 did not exist on the lower surface of the lightconducting plate 9. Light beam F1 shown in FIG. 5 indicates a beam whichmade incidence onto the light incident side surface 16 of the lightconducting plate 9 with an angle of incidence 90 degrees from its normalline, that is, its angle of refraction θ₁ equals the critical angle forthe total reflection inside the light conducting plate 9. If the indexof refraction for air is n₁ and that of the light conducting plate 9 isn₂, it is known that θ₁=sin=⁻¹(n₁/n₂), and the angle of incidence θ₂ ofthe beam F1 at the lower surface of the light conducting plate 9 isgiven by θ₂=90 degrees−θ₁. If the light conducting plate is ofpolycarbonate, n₂=1.59 and hence θ₁= 38.97 degrees and θ₂=51.03 degrees.Since this angle of incidence θ₂ is greater than the critical angle θ₁for total reflection, light beam F1 will undergo total reflection at thelower surface of the light conducting plate 9 if the diffusion layer 15is not present on the lower surface of the light conducting plate 9.Similarly, total reflection will take place also at the upper surface ofthe light conducting plate 9.

[0009] Consider another light beam F2 entering from the linear lightsource 7 into the light conducting plate 9. Since its angle ofrefraction θ₃ is smaller than θ₁, its angle of incidence θ₄ at the upperand lower surfaces of the light conducting plate 9 is larger than θ₂.Accordingly, light beam F2 from the linear light source 7 undergoestotal reflections at both upper and lower surfaces of the lightconducting plate 9 if there is no diffusion layer 15.

[0010] Since the reflecting plates 14 are provided on the other sidesurfaces of the light conducting plate 9 (that is, other than the lightincident side surface 16), light which is reflected on them is nearlyentirely reflected back into the interior of the light conducting plate9. Since the angle of incidence at the upper and lower surfaces does notchange by such reflections, light beam F2 continues to undergo totalreflection. Loss of light may be considered negligible by reflection bythe reflecting plates 14 made of a metallic dielectric material.

[0011] Next, consider the light source. If a cold cathode ray tube isused as the linear light source 7, the surface of the glass tube of sucha cold cathode ray tube is covered with a fluorescent layer having aproperty of total diffusion against light from outside. In other words,light which is made incident onto the linear light source 7 is reflectedtherefrom without any loss.

[0012] Thus, the light conducting plate 9 without the diffusion layer 15on its lower surface can seal in with a very high efficiency any lightwhich enters from the linear light source 7, but a plate which seals inincident light cannot serve as a light source. The sealed light must beallowed to come out through a light emitting surface 17 (the uppersurface of the light conducting plate 9). This is why the diffusionlayer 15 is provided on the lower surface of the light conducting plate9 such that light which is incident on the diffusion layer 15 isdiffused and that portion of the light which does not satisfy thecondition for total reflection is allowed to escape. This escapedportion of light is further diffused by the diffusion plate 11 on theupper surface of the light conducting plate 9.

[0013] In summary, light from the linear light source 7 is emitted witha very high efficiency towards the display surface of the LC displayapparatus. Even light coming from the display surface is similarlyre-emitted towards the display surface without any loss.

[0014] Diffusion of light from such an edge-light type surface lightsource device 6 is illustrated in FIG. 6. Light beam F3 reflected on thelower surface is diffused as Lambert beam, and the portion which doesnot satisfy the condition for total reflection is emitted out throughthe upper surface of the light conducting plate 9 as a semi-sphericalbeam F4. Light beam F4 is further diffused by the diffusion plate 11,becoming Lambert beam F5. This passes through the two converging lensplates 12 and 13 and is emitted upwards as beam F6.

[0015] When such a surface light source is used as a back-light sourcefor an LC display apparatus, however, the brightness is not sufficient.In order to increase the front brightness of an LC display apparatus, itis generally required that the direction of light emission from thesurface light source device 6 should be unidirectionally aligned.Another reason for low front brightness is the low opening ratio of theLCD panel. As shown generally at 21 in FIG. 7, the LCD panel has liquidcrystal 29 sealed between a glass plate 25 having thin-film transistors(TFT) 22, wiring 23 and a black matrix 24 formed on its upper surfaceand another glass plate 28 having a color filter 26 and a transparentelectrode 27 formed on its lower surface and polarization plates 30 and31 thereabove and therebelow. The areas covered by the black matrix 24serve to screen the light from the surface light source device 6, andonly the open areas 32 surrounded by the black matrix 24 allow the lightto pass through. Because the ratio of these openings is low, sufficientbrightness cannot be obtained on the display surface of the LC displayapparatus. If it is desired to make the image elements (pixels) verysmall in order to improve the image quality of the LCD panel 21, inparticular, the open areas 32 become small because there is a limit tohow small the black matrix 24 can be made.

[0016] One way to minimize the reduction in brightness due to the blackmatrix 24, as shown in FIG. 8, is to use a micro-lens array 33 to focusthe light emitted from the surface light source device 6 at the openareas 32 of the LCD panel 21 such that all light beams will pass throughthe openings. If there are fluctuations in the direction of light fromthe surface light source device 6, however, the micro-lens array 33cannot focus light at small open areas 32, and the brightness cannot besuccessfully made higher.

[0017]FIG. 9 shows the relationship between the angle of light emission(measured from a line perpendicular to the display surface of an LCdisplay apparatus) and brightness, Curve A indicating the brightness ofa pixel portion where the TFT is on and it is in the light-transmittingcondition and Curve B indicating the brightness of a pixel portion wherethe TFT is off and it is in the light-non-transmitting condition. Theangle of emission is defined negative on the side of the light source.FIG. 9 shows that the brightness-darkness contrast is great in thefrontal directions of the LC display apparatus but the lighttransmissivity is low and the contrast is poor in diagonal directions.If the display surface is looked at diagonally at a very large angle,the brightness-darkness contrast may be inverted or the displayed colormay appear differently.

[0018] When an LC display apparatus is used in a device to be looked atby many viewers such as a television set, it is necessary to make thedisplay surface visible also from directions other than the frontaldirection. Since LC display apparatus are not easily visible fromdiagonal directions, it may be considered feasible, as shown by brokenline in FIG. 8, to place a diffusion plate 34 on top of the LCD panel 21such that light emitted from the LCD panel 21 can be caused to propagatealso sideways. If such a diffusion plate 34 is installed on the side ofthe surface of the LCD panel 21, however, light-emitting points come tobe on the diffusion plate 34. Thus, if use is made of a surface lightsource device with fluctuations in the direction of light emission,beams of light which passed through mutually adjacent pixels may overlapeach other on the diffusion plate 34, resulting in a poorly focusedimage.

[0019] If a color filter is used in a color LC display apparatus, thebrightness of the display surface becomes lower because each pixelallows only light within a specified range of wavelength to pass and theamount of transmitted light becomes at most about one third of theamount of incident light.

[0020]FIG. 11 shows an attempt to solve this problem by dispersing thewhite light from the surface light source device 6 into red (R), green(G) and blue (B) colors by means of a diffraction grating 35 andfocusing light of each color by means of a micro-lens array 36. Thismethod can be successful, however, only if the beams of light emittedfrom the surface light source device 6 is unidirectionally aligned.

[0021] The polarization plates 30 and 31, which are disposed above andbelow, further serve to cut polarized light in one direction. Thus, theamount of transmitted light is further reduced by one half, furtherreducing the brightness of the display surface.

[0022] In view of the above, it has been suggested to make use of apolarization separator plate 37, as shown in FIG. 12, instead of thelower one of the polarization plates. Of the light beams emitted fromthe surface light source device 6, light beams polarized in a specifieddirection (referred to as the P-polarized light) can pass through boththe separation plate 37 and the upper polarization plate 30, but lightbeams polarized in the perpendicular direction (referred to as theS-polarized light) are reflected by the separation plate 37 and returnto the surface light source device 6. The returned S-polarized light isdiffused inside the surface light source device 6 and emitted again asunpolarized light. As this process is repeated, all light emitted fromthe surface light source device 6 is taken out as P-polarized light fromthe LCD panel 21. This method, too, requires that the light emitted fromthe surface light source device 6 be unidirectionally aligned.

[0023] In summary, in order to solve the problems of prior art surfacelight source devices such as low front brightness, lower brightness indiagonal directions, lowering of brightness due to the black matrix usedin the LCD panel and lowering of brightness due to a color filter ofpolarization plates, emitted light must be all in one direction.

[0024] In other words, light emitted from the surface light sourcedevice must be converged and collimated. As shown in FIG. 6, prior artedge-light type surface light source devices were provided with a pairof converging lens plates 12 and 13 to converge emitted light. Withprior art edge-light type surface light source devices, however, lightconvergence cannot be effected satisfactorily because the light which isemitted in all directions from the light-emitting surface 17 of thelight conducting plate 9 is once converted into Lambert beam by thediffusion plate 11 and this is then made convergent by means of theconverging lens plates 12 and 13. With prior art edge-light type surfacelight source devices, furthermore, the diffusion plate and theconverging lens plates are stacked on top of the light conducting plate9 such that a loss of light occurs also through these plates, adverselyaffecting the overall brightness. Many attempts have been made toimprove the brightness of LC display apparatus but none has so far beensatisfactory.

SUMMARY OF THE INVENTION

[0025] It is therefore an object of this invention in view of the aboveto provide a surface light source device with high directionality in theemitted light, capable of limiting the direction of emitted light withina narrow range.

[0026] It is another object of this invention to make use of such asurface light source device to convert wasteful light into useful lightto thereby improve the brightness of a LC display apparatus and toimprove its visibility, depending on the purpose of its use.

[0027] A surface light source device embodying this invention, withwhich the above and other objects can be accomplished, may becharacterized as comprising a light conducting plate with a light sourcedisposed adjacent to one of its side surfaces. At least one pattern isformed on the upper light emitting surface of the light conducting plateand/or the lower surface. The pattern is provided such that the sum ofaverage slope angles of the light emitting surface and the oppositesurface on a first sectional surface which is perpendicular to both thelight incident surface and the light emitting surface is greater thanthe sum of average slope angles of the light emitting surface and theopposite surface on a second sectional surface which is parallel to thelight incident side surface. Image display apparatus incorporating sucha surface light source device have improved brightness and othercharacteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The accompanying drawings, which are incorporated in and form apart of this specification, illustrate embodiments of the invention and,together with the description, serve to explain the principles of theinvention. In the drawings:

[0029]FIGS. 1A and 1B are respectively a front view and a sectional viewof a prior art surface light source device of a verticallydownward-facing type;

[0030]FIG. 2 is a partially broken diagonal view of an edge light typesurface light source device;

[0031]FIGS. 3A and 3B are examples of patterns on the diffusion layer;

[0032]FIG. 4A is a bottom view of a diffusion layer, and FIG. 4B is itssectional view taken along line 4B-4B of FIG. 4A;

[0033]FIG. 5 is a sectional view of the diffusion layer of FIG. 2 toshow its functions;

[0034]FIG. 6 is a sketch for showing the direction characteristics oflight emitted from the surface light device of FIG. 1;

[0035]FIG. 7 is an exploded diagonal view of a liquid crystal panel;

[0036]FIG. 8 is a schematic sectional view of a structure for reducinglowering of brightness due to black matrix in an image displayapparatus;

[0037]FIG. 9 is a graph which shows the relationship between brightnessand angle of light emitted from the display surface of an image displayapparatus;

[0038]FIGS. 10, 11 and 12 are schematic drawings for explaining variousprior art attempts to increase the brightness of the display surface ofan image display apparatus;

[0039]FIG. 13 is a diagonal view of an edge-light type surface lightsource device according to one embodiment of this invention;

[0040]FIG. 14 is a drawing for showing the effect of the deflectionpattern on the device of FIG. 13;

[0041]FIGS. 15A, 15B and 15C show directions of light deflected by thedeflection pattern shown in FIG. 14 and converged by the lightconverging pattern of FIG. 13;

[0042]FIG. 16 is a diagonal view of another surface light source deviceembodying this invention with a different linear light source;

[0043]FIG. 17 is a diagonal view of a surface light source device withlight converging and deflection patterns of specific designs;

[0044]FIG. 18 is a portion of a sectional view taken along line 18-18 ofFIG. 17;

[0045]FIG. 19 is a diagonal view of another surface light source devicewith light converging and deflection patterns with different designs;

[0046]FIG. 20A is a portion of the lower surface of the light conductingplate of FIG. 19, and FIG. 20A is a portion of its sectional view takenalong line 20B-20B;

[0047]FIG. 21 is a sectional view of FIG. 20B with light reflected bythe deflection pattern;

[0048]FIGS. 22A and 22B are drawings for defining average slope angles;

[0049]FIGS. 23A, 23B, 23C and 23D are drawings for showing average slopeangles of various patterns;

[0050]FIG. 24 is a drawing for showing directions of light passingthrough a light conducting plate;

[0051]FIG. 25 is a light direction trajectory diagram for FIG. 24;

[0052]FIG. 26 is a diagonal view of a surface light source device havingpatterns with the average slope angle 0 in the Y-direction;

[0053]FIG. 27 is a light direction trajectory diagram for the device ofFIG. 26;

[0054]FIG. 28 is a diagonal view of a surface light source device havingrandom patterns with approximately equal average slope angles in the X-and Y-directions;

[0055]FIG. 29 is a light direction trajectory diagram for the device ofFIG. 28;

[0056]FIG. 30 is a diagonal view of a surface light source device ofwhich the average slope angle in the Y-direction is larger than that inthe X-direction;

[0057]FIG. 31 is a light direction trajectory diagram for the device ofFIG. 30;

[0058]FIG. 32A is a sectional view of a portion of a light conductingplate having a pattern according to an undesirable design with atrajectory of light beam therethrough, and FIG. 32B is a light directiontrajectory diagram therefor;

[0059]FIG. 33 is a sectional view of a portion of a light conductingplate having a pattern according to a preferred design with a trajectoryof light beam therethrough;

[0060]FIG. 34 is a diagonal view of a surface light source device havinga reflecting plate on the lower surface;

[0061]FIG. 35 is a diagonal view of another surface light source deviceembodying this invention;

[0062]FIG. 36 is a diagonal view of another surface light source devicehaving a prism sheet;

[0063]FIGS. 37 and 38 are diagonal views of other surface light sourcedevices embodying this invention;

[0064]FIGS. 39A and 39B are a diagonal view and a front view,respectively, of ranges of light directions when there is a lightconverging pattern on the upper surface of the light conducting plate;

[0065]FIG. 40 is a diagonal view of another surface light source deviceembodying this invention;

[0066]FIG. 41 is a sectional view of a portion of a light conductingplate for showing the function of a diffusion pattern;

[0067]FIG. 42 is a sectional side view of a portion of a lightconducting plate with triangular deflection pattern on its lowersurface;

[0068]FIG. 43 is a portion of FIG. 42 to show the effects of reflection;

[0069]FIG. 44 is a light direction trajectory diagram corresponding tothe deflection pattern of FIG. 43;

[0070]FIG. 45 is a sectional side view of a portion of another lightconducting plate with V-shaped grooves;

[0071]FIG. 46 is a light direction trajectory diagram corresponding tothe pattern shown in FIG. 45;

[0072]FIG. 47 is a diagonal view of another surface light source deviceembodying this invention;

[0073]FIG. 48 is a diagonal view of still another surface light sourcedevice embodying this invention;

[0074]FIG. 49 is a sectional view for showing reflection of light byside surface reflecting plates of FIG. 48;

[0075]FIG. 50 is another sectional view for showing reflection andalignment of light in the device of FIG. 48;

[0076]FIG. 51 is a diagonal view of another surface light source deviceembodying this invention with a wedge-shaped light conducting plate;

[0077]FIG. 52 is a schematic sectional view of a light conducting plateembodying this invention;

[0078]FIG. 53 is a sectional view of a portion of a light conductingplate for showing how a dark area can occur;

[0079]FIGS. 54 and 55 are sectional views of light incident surfaces forreducing the dark area of a surface light source device;

[0080]FIG. 56 is a diagonal view of still another surface light sourcedevice embodying this invention having an L-shaped light source;

[0081]FIG. 57 is a plan view of the device of FIG. 56;

[0082]FIG. 58 is a plan view of another surface light source device withan L-shaped light source;

[0083]FIGS. 59, 60 and 61 are drawing of other surface light sourcedevices with an L-shaped light source;

[0084]FIG. 62 is a sectional side view of a surface light source devicewith a prism sheet;

[0085]FIG. 63 is a sectional view of a prism sheet for showingdeflection of light by such a prism sheet;

[0086]FIG. 64 is a sectional view of another prism sheet with adifferent prism pattern;

[0087]FIG. 65 is a sectional view of still another prism sheet with astill another prism pattern;

[0088]FIG. 66 is a side view of a surface light source device with aprism sheet with a different prism pattern;

[0089]FIG. 67 is a side view of a surface light source device havingboth a prism sheet and side surface reflecting plates;

[0090]FIG. 68 is an exploded diagonal view of an image display apparatusembodying this invention;

[0091]FIG. 69 is a drawing for explaining the principles of the imagedisplay apparatus of FIG. 68;

[0092]FIG. 70 is a drawing for explaining the principles of anotherimage display apparatus embodying this invention;

[0093]FIG. 71 is a schematic drawing of a color image display apparatusembodying this invention;

[0094]FIG. 72 is a schematic drawing of another color image displayapparatus embodying this invention;

[0095]FIG. 73 is a schematic drawing of still another image displayapparatus embodying this invention using a polarization separationelement;

[0096]FIG. 74 is an LC television set embodying this invention;

[0097]FIG. 75 is a diagonal view of the image display apparatuscontained in the LC television set of FIG. 74;

[0098]FIG. 76 is a schematic drawing of another image display apparatushaving a diffusion plate;

[0099]FIGS. 77A and 77B are drawings of diffusion plates havingdiffusion surface on the upper surface and lower surface, respectively;

[0100]FIG. 78 is a schematic drawing of an image display apparatus witha diffusion surface formed on its LCD panel;

[0101]FIG. 79 is a schematic drawing of an image display apparatus usinga prior art LCD panel;

[0102]FIGS. 80, 81, 82 and 83 are schematic drawings of image displayapparatus according to different embodiments of this invention;

[0103]FIGS. 84, 85 and 86 are schematic drawings of LCD panels for imagedisplay apparatus according to different embodiments of the invention;

[0104]FIG. 87 is a schematic diagonal view of an automatic tellermachine (ATM) using an image displaying apparatus embodying thisinvention;

[0105]FIGS. 88 and 89 are schematic drawings of directions of lightemitted from the image displaying apparatus of FIG. 87;

[0106]FIGS. 90, 91 and 92 are schematic drawings of image displayingapparatus for the ATM of FIG. 87 according to different embodiments ofthe invention;

[0107]FIG. 93 is a schematic plan view of a car with an automaticnavigation system;

[0108]FIGS. 94A, 94B and 94C are schematic drawings of image displayapparatus with different structures for the system of FIG. 93; and

[0109]FIG. 95 is a schematic drawing of the interior of a train car withan LC display.

[0110] Throughout herein, components which are equivalent orsubstantially similar are indicated by the same numbers and notnecessarily described or explained repetitiously. In all embodiments ofthe invention, an XYZ-coordinate system is defined in the same mannerrelative to each surface light source device embodying this inventionand hence is not necessarily defined with respect to each drawing.

DETAILED DESCRIPTION OF THE INVENTION

[0111]FIG. 13 shows the principle of an edge-light type surface lightsource device 51 according to one embodiment of this invention, having alight conducting plate 52 of a rectangular planar shape made of amaterial which is optically transparent and has a large index ofrefraction (>1). A linear light source 54 such as a cold cathode raytube and a reflector 55 surrounding the linear light source 54 aredisposed on the light-incident side surface 53 of the light conductingplate 52, the upper surface of the light conducting plate 52 serving asthe light emitting surface 56. A deflection pattern 57 is provided onthe lower surface of the light conducting plate 52, and a lightconverging pattern 58 is provided on the (upper) light emitting surface56. By the deflection pattern 57 is meant an optical pattern whichserves, when light inside the light conducting plate 52 is reflected, tochange the direction of at least a portion of the reflected light suchthat the angle made with that portion of light with the light emittingsurface 56 or the opposite (lower) surface becomes slightly larger thanbefore, or that the angle with the direction perpendicular to the lightemitting surface 56 or the opposite (lower) surface becomes slightlysmaller. It is particularly desirable that reflection takes place suchthat, when projected on a plane perpendicular to both the light-incidentside surface 53 and the light emitting surface 56, the angle to thedirection perpendicular to the light emitting surface 56 becomesslightly smaller. By the light converging pattern 58 is meant anotheroptical pattern adapted to converge light emitted from the lightconducting plate 52 into the direction of a plane which is perpendicularto both the light-incident side surface 53 and the light emittingsurface 56. For the convenience of explanation which follows, thedirection which is perpendicular to the light-incident side surface 53will be defined as the X-axis, the direction which is perpendicular tothe light emitting surface 56 of the light conducting plate 52 will bedefined as the Z-direction, and a direction perpendicular to both theX-axis and the Z-axis will be defined as the Y-axis.

[0112]FIG. 14 shows how the deflection pattern 57 functions, as seenalong the Y-axis. Light beam F emitted from the linear light source 54enters the light conducting plate 52 through its light-incident sidesurface 53 either directly or after being reflected by the reflector 55.The light F which makes incidence on the lower surface of the lightconducting plate 52 at the critical angle ø of total reflection isreflected by the deflection pattern 57 with an angle of reflection ø-δ,which is slightly smaller than the critical angle ø, to be madeincidence on the upper surface of the light conducting plate 52. Thelight which is made incidence on the upper surface at the angle ofincidence of ø-δ is emitted out of the light conducting plate 52 nearlyparallel to the upper surface of the light conducting plate 52. Lightwhich is reflected on the lower surface of the light conducting plate 52at an angle of reflection greater than the critical angle ø is totallyreflected on the upper surface of the light conducting plate 52 anddirected again towards the lower surface of the light conducting plate52. If it is then reflected by the deflection pattern 57 with an angleof reflection smaller than the critical angle ø, it will be emitted outthrough the upper surface nearly parallel thereto. With this processrepeated, light is emitted from the entirety of the upper surface of thelight conducting plate 52 within a narrow range (shown shaded in FIG.14) nearly parallel to the upper surface. If the deflection angle δ issufficiently small, the emitted light from the upper surface of thelight conducting plate 52 becomes nearly parallel to the light emittingupper surface 56 of the light conducting plate 52.

[0113] Directions of light deflected by the deflection pattern 57 andemitted from the upper surface of the light conducting plate 52 areshown in FIGS. 15A and 15B as seen respectively on the ZX-plane and onthe XY-plane. FIG. 15C shows the directions of emitted light as seen onthe XY-plane after converged by the light converging pattern 58. Asdescribed above, light which is reflected by the deflection pattern 57on the lower surface of the light conducting plate 52 is partly emittedout nearly parallel to the upper surface of the light conducting plate52 as shown in FIG. 15A but, if seen on the XY-plane, it spreads over arange of 180 degrees, as shown in FIG. 15B. When light is emitted outfrom the upper surface of the light conducting plate 52, however, itpasses through the light converging pattern 58 and is thereby convergedon the ZX-plane (in the X-direction). As a result, light is emitted fromthe upper surface of the light conducting plate 52 only in the directionof the X-axis.

[0114] Although FIG. 13 shows an embodiment according to which the lightconverging pattern 58 is provided on the light emitting surface 56 ofthe light conducting plate 52 and the deflection pattern 57 on theopposite lower surface, the deflection pattern 57 may be provided on thelight emitting surface 56 with the light converging pattern 58 providedon the opposite lower surface. Alternatively, both the light convergingpattern 58 and the deflection pattern 57 may be provided together on thelight emitting surface 56 of the light conducting plate 52 or on theopposite lower surface.

[0115]FIG. 16 shows another surface light source device 59 provided witha linear light source 54 of a different structure, having point lightsources 54 a such as light-emitting diodes arranged in a linear array.The point light sources 54 a may be arranged in a plurality of rows.

[0116]FIG. 17 shows a surface light source device 60 having a lightconverging pattern 58 and a deflection pattern 57 with specific designs.The light converging pattern 58, according to this embodiment of theinvention, is uniform in the direction of the X-axis and shaped liketriangular prisms with triangular cross-sectional shapes covering theentire light emitting surface 56 of the light conducting plate 52. Thedeflection pattern 57 is uniform in the direction of the Y-axis withtriangular cross-sectional shapes, covering the entire lower surface ofthe light conducting plate 52. This deflection pattern 57 serves toreflect light on the slopes facing the light source such that theemitted light will be nearly parallel to the light emitting surface 58.FIG. 18 shows how light is reflected by the deflection pattern 57 and isemitted out through the light emitting surface 56. (FIG. 14 should bereferenced for a view along the Y-axis.) A portion of the light emittedthus from the light conducting plate 52 is refracted by the prismportion 58 a of the light converging pattern 58 and converged in a planeparallel to the ZX-plane, providing a unidirectional beam approximatelyin the direction of the X-axis. A portion of light emitted from theprism portion 58 a sideways is directed again by the neighboring prismportion 58 a into the interior of the light conducting plate 52 and istotally reflected at the inner surface of this neighboring prism portion58 a, returning again to the lower surface of the light conducting plate52. The light thus returning to the lower surface is repeatedlyreflected and is emitted out through the light emitting surface 58.Thus, light which is emitted from the light emitting surface 58 nearlyparallel to the XY-plane is converged to the ZX-plane, thereby becomingoriented in the direction of the X-axis. The portion of light which hasnot been converged returns back into the light conducting plate 52 andthen emitted out. In other words, light is utilized with a highefficiency. As can be understood from this particular embodiment, thedeflection pattern 57 need not be able to deflect all reflected lightinto a direction perpendicular to the light emitting surface 56. It issufficient if it can deflect at least a portion of the lightperpendicularly to the light emitting surface 56.

[0117]FIG. 19 shows another surface light source device 61 with lightconverging and deflection patterns 58 and 57 with still differentpatterns shown in FIGS. 20A and 20B. The light converging pattern 58according to this embodiment of the invention is uniform in theX-direction and formed as a series of cylindrical lenses. The deflectionpattern 57 comprises a plurality of rows of cylindrical lenses in thedirection of the X-axis and matching the focus of the light convergingpattern 58.

[0118] Since the light conducting plate 52 can seal in light veryefficiently, as explained above, light inside the light conducting plate52 is reflected repeatedly and emitted out when reflected by thedeflection pattern 57 such that the angle of incidence becomes smallerthan the critical angle ø for total reflection. In this situation, sincelight is reflected near a focus of a cylindrical lens in the lightconverging pattern 58, the emitted light is converged in the ZX-plane asshown in FIG. 21, and light is eventually emitted nearly in thedirection of the X-axis.

[0119] The light converging pattern 58 and the deflection pattern 57described above can be correlated as follows. The average slope angleθx* of the deflection pattern 57 is defined as follows:

Σ|(θxi) (Δxi)|/Σ(Δxi)|

[0120] where (θxi) (i being a dummy index i=1, 2, . . . ) is the angleof each part with respect to the X-axis, (ΔXi) is the length of thecorresponding base, as shown in FIG. 22A, and Σ indicates summation overall dummy indices “i”.

[0121] Similarly, the average slope angle θy* of the deflection pattern57 is defined as follows:

Σ|(θyj)(ΔYj)|/Σ|(ΔYj)|

[0122] where (θyj) (j being a dummy index j=1, 2, . . . ) is the angleof each part with respect to the Y-axis, (ΔYj) is the length of thecorresponding base, as shown in FIG. 22B, and Σ indicates summation overall dummy indices “j”.

[0123]FIGS. 23A, 23B, 23C and 23D show the average slope angles θ* forvarious patterns serving as examples. FIG. 23A shows sawtooth patternwith slope of 5 degrees, FIG. 23B shows another sawtooth pattern withslope of 3 degrees, and their average slope angles are respectively θ*=5degrees and θ*=3 degrees. FIG. 23C is a triangular wave pattern with afirst slope portion with slope of 5 degrees (with the length ofbase=3Λ/8 where Λ is the pitch) and a second slope portion with slope of3 degrees with the length of base=5Λ/8) and its average slope angle isθ*=3.75 degrees. FIG. 23D is a pattern with V-shaped grooves withaverage slope angle θ*=3 degrees. The average slope angle of a flatsurface is 0 degrees.

[0124] In terms of the average slope angles θx* and θy* thus defined forthe deflection pattern 57 and the light converging pattern 58,respectively, the required relationship between the deflection pattern57 and the light converging pattern 58 may be described that the averageslope angle of the light converging pattern θy* should be greater thanthe average slope angle of the deflection pattern θx*.

[0125] This concept of average slope angle can be extended to patternsof other types. When a pattern curve is given, it may be approximated bya series of line segments, the average slope angle of these linesegments is calculated, the length of each line segment is made toapproach zero, and the limit to which the average slope angle approachesmay be defined as the average slope angle of this pattern curve. In thecase of a cylindrical lens pattern as shown in FIG. 19, it may beapproximated by a triangular pattern obtained by tangents at both sides,and the average slope angle of this triangular pattern may be obtained.In the case of a fine random diffusion pattern, it is possible tocorrelate the roughness with the average slope angle.

[0126] The patterns which are provided to a light conducting plate ofthis invention can be generalized by using the concept of average slopeangle. Since the deflection pattern 57 and the light converging pattern58, in particular, look alike or are made on the same surface, they aresometimes difficult to distinguish. It is therefore meaningful tointroduce the concept of average slope angle to generalize thedescription therefor. In other words, the surface light source deviceaccording to this invention can be characterized wherein the sum θy*(hereinafter sometimes referred to as the average slope angle in theY-direction) of the average slope angle of the pattern on the lightemitting surface 56 on a sectional surface parallel to the lightincident side surface 53 (a sectional surface parallel to the YZ-plane)and the average slope angle of the pattern on the lower surface oppositethereto should be larger than the sum θx* (hereinafter sometime referredto as the average slope angle in the X-direction) of the average slopeangle of the pattern on the light emitting surface 56 on a sectionalsurface perpendicular to the light incident side surface 53 (a sectionalsurface parallel to the ZX-plane) and the average slope angle of thepattern on the lower surface opposite thereto, that is, (the averageslope angle in the Y-direction, or θy*)>(average slope angle in theY-direction, or θx*).

[0127] Next will be explained why the patterns on the light conductingplate 52 can be generalized as described above. In other words, it willbe shown next how the light entering from the linear light source 54should change its direction inside the light conducting plate 52 suchthat the emitted light becomes unidirectional. In FIG. 24, it will beassumed that the linear light source 54 is on the left-hand side (thelight incident side surface 53) of the light conducting plate 52. Thedirection of light emitted at any point P1 of the linear light source 54is within a range given by the right-hand hemisphere R1. When a lightbeam in this range reaches a point P2 on the left-hand side surface ofthe light conducting plate 52, the range R2 of direction of therefracted light is determined by the Snell's law of refraction.

[0128] In FIG. 24, R3 indicates the range of direction of light which isreflected at any point Q1 on the lower surface of the light conductingplate 52 and can be emitted out of the light conducting plate 52. Thedirection in which any of such light can be emitted out from any pointQ2 on the upper surface of the light conducting plate 52 is indicated byR4, determined again by the Snell's law of refraction.

[0129] In order that the light emitted from the light conducting plate52 be all in one direction, the direction of light emitted from point Q2must be within a range r4 smaller than R4. Let r3 be the correspondingrange of direction of light from point Q1. In other words, the emittedlight becomes unidirectional if the light with direction within range R2is converted into light with direction within range r3. Now, in order toconsider the relationship between ranges R2 and r3, points P1, P2 and Q1are shifted to a common origin and ranges R1, R2 and R3 are superposed,as shown in FIG. 25. It will be sufficient if light beams inside thelight conducting plate 52 with directions within range R2 (as shown inFIG. 25) move into range r3 after repeating reflections inside the lightconducting plate 52. In what follows, diagrams like this (hereinafterreferred to as light direction trajectory diagrams) will be used tostudy the relationship between average slope angles and directionalityof emitted light.

[0130]FIG. 26 shows a surface light source device 62 of which theaverage slope angle θx* in the X-direction is finite and the averageslope angle θy* in the Y-direction is zero. This may correspond, forexample, to having on the lower surface of the light conducting plate 52a pattern 65 a which is uniform in the Y-direction, the upper surfacebeing flat. The light direction trajectory diagram of this surface lightsource device 62 is shown in FIG. 27, wherein the change in thedirection of a light beam is indicated by black circles. Any light beamwhich was initially in range R2 inside the light conducting plate 52 isnot diffused or deflected in the Y-direction but moves in theX-direction every time it is reflected on the lower surface of the lightconducting plate 52, eventually entering range R3 and then emitted outthrough the upper surface of the light conducting plate 52. Thus, lightbeams which are emitted out of the light conducting plate 52 are insidethe shaded portion of range R3 and, since there is no convergence in theY-direction, the emitted light will spread in Y-direction as shown byarrows in FIG. 26.

[0131]FIG. 28 shows a surface light source device 63 of which theaverage slope angle θx* in the X-direction is about equal to the averageslope angle θy* in the Y-direction (that is, θx*≈θy*>0). This maycorrespond, for example, to having a random diffusion pattern 65 b onthe lower surface of the light conducting plate 52, the upper surfacebeing flat. The light direction trajectory diagram of this surface lightsource device 63 is shown in FIG. 29. Light beams, which were initiallyin range R2 inside the light conducting plate 52, change directions atrandom and are emitted out of the light conducting plate 52 afterentering range R3. Thus, light beams will spread both in the X- andY-directions. If the diffusion due to the pattern is great, they will bespread all over range R3, as in the case of prior art diffusion layer.As a result, the emitted light will spread at random at shown by thearrows in FIG. 28.

[0132]FIG. 30 shows a surface light source device 64 of which theaverage slope angle θy* in the Y-direction is larger than the averageslope angle θx* in the X-direction (that is, 0<θx* “ θy*). This maycorrespond, for example, to having formed on the lower surface of thelight conducting plate 52 a pattern 65 c with stronger diffusion in theY-direction than in the X-direction, the upper surface being flat. Thelight direction trajectory diagram of this surface light source device64 is shown in FIG. 31. As shown in FIG. 31, a light beam which wasinitially in range R2 inside the light conducting plate 52 changes itsdirection significantly more along the Y-axis while its directionchanges only a little along the X-axis. In other words, the light beammoves many times in the Y-direction and finally enters range r3 whilemoving violently in the Y-direction, being emitted into range r4 (shownin FIG. 24). Thus, light beams emitted from the upper surface of thelight conducting plate 52 are nearly completely aligned along theX-direction as shown by the arrows in FIG. 30.

[0133] As shown above by way of examples, light beams well aligned inthe X-direction can be obtained if a pattern is formed on either theupper or lower surface (or both) of the light conducting plate 52 suchthat the average slope angle θy* on a sectional surface parallel to theYZ-plane is larger than the average slope angle θx* on a sectionalsurface parallel to the ZX-plane (especially if the latter is madesmall).

[0134] With reference to FIG. 32A, it is preferable that the pitch Λ ofthe pattern 66 (as seen sectionally parallel to the light-incident sidesurface 53 of the light conducting plate 52) be less than {fraction(1/10)}of the thickness (measured at the thickest position) T of thelight conducting plate 52. If the pitch Λ is too large, as shown in FIG.32A, some light beams will continue to be reflected nearly at the sameposition, giving rise to unevenness in light intensity. As shown by itscorresponding light direction trajectory diagram in FIG. 32B, lightbeams fail to oscillate in the Y-direction correctly, as indicated inFIG. 31, failing to be aligned. If the pitch is less than one tenth ofthe thickness, light beams spread in the Y-direction, as shown in FIG.33, giving rise to no unevenness.

[0135]FIG. 34 shows a surface light source device 67 having a reflectingplate 68 on the lower surface of its light conducting plate 52 forreflecting a majority of light for improving efficiency in the use oflight. If use is made of a diffusive reflecting plate instead, the angleof reflection is not always the same as the angle of incidence, and thisgives rise to light beams which fail to fall into region r3. It istherefore preferred to make use of a reflecting plate capable of causingnormal reflections.

[0136] The patterns on the light conducting plate 52 may be provided onboth of its surfaces, only on its upper surface or only on its lowersurface, as long as the average slope angle θy* in the Y-direction isgreater than that in the X-direction θx*. It is difficult, however, toprovide both a pattern which is uniform in the X-direction (such as thelight converging pattern 58) and a pattern which is uniform in theY-direction (such as the deflection pattern 57) on the same surfacebecause it would involve a two-dimensional fabrication processes (asshown in FIG. 38). In order to reduce the production cost, the lightconverging pattern 58 may be provided on the upper surface with thedeflection pattern 57 formed on the lower surface, as shown in FIGS. 19and 34. FIG. 35 shows another surface light source device 69alternatively provided with its light converging pattern 58 on the lowersurface and a deflection pattern 57 on the upper surface.

[0137] A prism sheet 70, which is uniform in the Y-direction, may beplaced on the light conducting plate 52, as shown in FIG. 36, in orderto change the direction of light beams from the X-direction to theZ-direction. If the pitch of the prism sheet 70 and that of thedeflection pattern are nearly the same, Moire fringes may appear. It isknown that Moire fringes become vague and small in such a situation ifthe distance between the deflection pattern 57 and the prism sheet 70 isincreased. Thus, it is preferred that the deflection pattern 57 beformed on the lower surface and the light converging pattern 58 on theupper surface.

[0138]FIG. 37 shows a surface light source device 71 having a pattern72, combining a deflection pattern 57 and a light converging pattern 58together, formed on the lower surface of its light conducting plate 52.This pattern 72 is characterized wherein its section in the ZX-directionis a deflection pattern 57 with a small average slope angle θx* and itssection in the YZ-direction is a light converging pattern 58 with arelatively large average slope angle θy*.

[0139] If a pattern as shown at 72 is to be formed on only one of thesurfaces of the light conducting plate 52, it is preferred to form thepattern on the lower surface of the light conducting plate 52 in view ofthe possibility of placing a prism sheet as shown in FIG. 36.

[0140] Such a pattern 72 may be formed on the upper surface of the lightconducting plate 52 as shown in FIG. 38 (illustrating another surfacelight source device 73). In this application, it is preferable to placea reflecting plate 68 for causing normal reflections on the lowersurface of the light conducting plate 52 in order to prevent light fromescaping therethrough.

[0141] The placing of a pattern in the direction of the X-axis (such asthe deflection pattern 55) on the upper surface of the light conductingplate 52 does not significantly affect the characteristic of the surfacelight source device 73 because the average slope angle θx* in thedirection of the X-axis. If a pattern in the direction of the Y-axis(such as the light converging pattern 58) is on the upper surface, onthe other hand, the slope in the pattern causes the light in range r3into two ranges r4 and r5 as shown in FIGS. 39A and 39B. The light inrange r5 is returned back into the light conducting plate 52, and onlythe light in range r4 remains.

[0142]FIG. 40 shows still another surface light source device 74embodying this invention, having a light converging pattern 58 uniformin the X-direction formed on the upper surface of its light conductingplate 52 and a small diffusion pattern 75 with nearly equal averageslope angles in any sectional surface (θx*≈θy*) on the lower surface.This diffusion pattern 75 is a random pattern which may be produced byproviding small indentations and protrusions or by coating with a paintwhich has light-diffusing property. It is different from prior artdiffusion layers in that the average slope angle is much smaller. Insummary, the lower surface of the light conducting plate 52 is randombut more gently uneven.

[0143] An incident beam of light onto the lower surface of the lightconducting plate 52 at the critical angle ø of total reflection isreflected by the diffusion pattern 75, as shown in FIG. 41, with a smallangle of scattering around the critical angle ø. Light exceeding thecritical angle ø is emitted out from the light emitting surface 56nearly parallel to the upper surface of the light conducting plate 52.By a prior art diffusion layer, by contrast, light is emitted in alldirections over a range of 180 degrees because the scattering takesplace in a wider range of directions.

[0144] Explained in terms of average slope angles, since the averageslope angle in the X-direction at the lower surface (θxd*) is equal tothe average slope angle in the Y-direction at the upper surface (θyd*),and since the average slope angle in the X-direction at the uppersurface (θxu*) is zero, the sum of the average slope angles in theX-direction at the upper surface and the lower surface (θx*) is equal toθxd*. The sum of the average slope angles in the Y-direction at theupper and lower surfaces (θy*=θyu*+θyd) is greater than θx*.

[0145]FIG. 42 shows a preferable pattern 76 in a sectional surfaceparallel to the ZX-plane (called the X-direction pattern). This is likethe deflection pattern 57 shown in FIG. 19. The degree of diffusion bythe X-direction pattern 76 may be considered equal to the average ofchanges in direction by each reflection. In other words, a certain levelof degree of diffusion is necessary in order to cause as much light fromthe linear light source 54 as possible to be emitted from the upperlight emitting surface 56 of the light conducting plate 52.

[0146] Consider, for example, an X-direction pattern (shown in FIG. 43at 76) with triangular wave shape and another X-direction pattern (shownin FIG. 45 at 77) with V-shaped grooves or mesa-shaped hills, bothhaving the same average slope angle θx* (=θx). Although these twopatterns 76 and 77 have the same degree of diffusion, the first pattern76 causes about ½of light to change direction by 2θx (as shown in FIG.43), while the second pattern 77 causes about ¼of light to changedirection by 4θx. In other words, both of these patterns 76 and 77 havethe same degree of diffusion, causing the same amount of light to beemitted from the upper surface of the light conducting plate 52. Asshown in the light direction trajectory diagram of FIG. 44 (whereincircled numbers show the sequence of shifts by light), however, thedirection of light changes by the first pattern 76 at approximatelyequal intervals such that light is sure to move into region r3 and beemitted from the small range r4, while, as shown by the light directiontrajectory diagram of FIG. 46, the second pattern of FIG. 45 causeslight to move a great deal some times while light direction may not moveat some other times, such that it may jump over the range r3. In otherwords, the triangular wave pattern 76 is more effective in aligninglight emitted from the light conducting plate 52.

[0147]FIG. 47 shows still another surface light source device 78 havinga reflecting plate 68 on the lower surface of its light conducting plate52 and a diffusion pattern 79, uniform in the X-axis perpendicular toits light incident side surface 53, formed on the upper surface of thisreflecting plate 68. The shape of the diffusion pattern 79 does notlimit the scope of this invention. Light escaping from the lower surfaceof the light conducting plate 52 is reflected by the reflecting plate68, and as it returns into the light conducting plate 52, it is diffusedin the direction of the Y-axis due to the diffusion pattern 79 on thereflecting plate 68. This enhances the diffusion of light in thedirection of Y-axis inside the light conducting plate 52, improving theefficiency of bringing the light beams into region r4 (shown in FIG.31).

[0148]FIG. 48 shows still another surface light source device 80embodying this invention, having side surface reflecting plates 81 onall side surfaces of its light conducting plate 52 except the lightincident side surface 53 adjacent to the linear light source 54. Asshown in FIG. 49, light beams reflecting inside the light conductingplate 52 and escaping through its side surfaces are reflected by theseside surface reflecting plates 81 and return into the light conductingplate 52. Accordingly, the loss of light can be reduced by thisembodiment and more light can be emitted from the light emitting surface56. Since these side surface reflecting plates 81 are intended to causenormal reflections, they can be designed easily. With the side surfacereflecting plates 81 thus provided, it is preferable that theX-direction pattern 82 (or the deflection pattern 57) be symmetric inthe X-direction, as shown in FIG. 50, such that not only can light fromthe linear light source 54 be unidirectionally emitted from the lightemitting surface 56, but light reflected by the side surface reflectingplates 81 and returned back into the light conducting plate 52 can alsobe aligned and emitted from the light emitting surface 56, therebyimproving the emission efficiency.

[0149]FIG. 51 shows still another surface light source device 83embodying this invention, using a wedge-shaped light conducting plate52. A light converging pattern 58, uniform in the direction of theX-axis, is formed on the upper surface of the light conducting plate 52,and the sloped lower surface is smooth and flat. With such awedge-shaped light conducting plate 52, the wedge angle, or the anglebetween the upper and lower surfaces of the light conducting plate 52,may be considered the average slope angle. With this surface lightsource device 83, as light continues to undergo total reflections on thesloped lower surface, its angle with the line perpendicular to the lightemitting surface 56 becomes gradually smaller. As soon as it becomessmaller than the critical angle of total reflection, it is emitted outof the light emitting surface 56 with a small upward angle with theupper surface of the light conducting plate 52, and aligned in theY-direction by means of the light converging pattern 58. In summary,aligned light in the direction of the X-axis is emitted from the lightemitting surface 56. Since the light conducting plate 52 according tothis embodiment does not have any side surface opposite to the lightincident side surface 53, light which enters the light conducting plate52 can be emitted through the light emitting surface 56 at a highemission efficiency. Reflecting plates may be installed on the lowerand/or side surfaces of the light conducting plate 52 for normalreflection.

[0150]FIG. 52 shows still another surface light source device embodyingthis invention, having a sawtooth deflection pattern on the lowersurface of the light conducting plate 52, the slopes 57 a of thesawtooth pattern facing in the direction of the linear light source 54.With a deflection pattern thus designed, the area of surface forreflection is increased such that more light can be emitted from thelight emitting surface 56. This design is particularly useful when noside surface reflecting plates are used.

[0151] If the light incident side surface 53 of the light conductingplate 52 is perpendicular to the light emitting surface 56 and flat, asshown in FIG. 53, there is a portion of the light emitting surface 56which cannot be reached even by light which enters the light conductingplate with a largest possible angle of refraction β. This portion ishereinafter referred to as the dark area D. One method of reducing thedark area D is to provide a V-shaped groove 85 on the light incidentside surface 53 of the light conducting plate as shown in FIG. 54 suchthat the light incident side surface 53 is effectively no longerperpendicular to the light emitting surface 56. Another method is, asshown in FIG. 55, to provide an optical pattern 86 on the light incidentside surface 53 such that light can enter through the light incidentside surface 53 at a larger angle.

[0152]FIGS. 56 and 57 show another surface light source device 87embodying this invention having a linear light source 54 which isL-shaped. For this reason, a single light conducting plate cannotprovide the kinds of patterns required by the present invention. Thus, aquadrangular planar plate is formed by combining two right-triangularplanar light conducting plates 52 a and 52 b together. The lightincident side surface 53 for each corresponding one of the lightconducting plates 52 a and 52 b is opposite to one of the arms of theL-shaped linear light source 54. The light converging pattern 58 on theupper surface is uniform in the direction perpendicular to each lightincident side surface 53. The deflection pattern 57 on the lower surfaceis uniform in the direction parallel to each light incident side surface53. Each of the light conducting plates 52 a and 52 b emits lightunidirectionally such that, as the device 87 as a whole, light isemitted in two mutually perpendicular directions as shown by the arrowsin FIG. 56. With these two triangular plates 52 a and 52 b, however, theboundary surface 88 is oblique to both light incident side surfaces 53.As shown by a broken line in FIG. 57, light reflected by the boundarysurface 88 changes its direction significantly and such reflected lightwill not reach the range r3 to be emitted outside. Because this willadversely affect the directionality of the device, it is preferable toapply a light absorbing material at the boundary surface 88.

[0153]FIG. 58 shows another surface light source device 89 using anL-shaped linear light source 54, characterized wherein the twotriangular light conducting plates 52 a and 52 b are combined togethersuch that their boundary surface 88 is step-wise. Thus, all sidesurfaces are either parallel or perpendicular to the corresponding lightincident side surface 53. Thus, light reflected by the boundary surface88 does not change the direction and the emission efficiency can beimproved. It is preferred to provide mirror surfaces at the boundary orto apply a light-absorbing paint on the boundary surface 88 so as toprevent light traveling from one to the other of the light conductingplates 52 a and 52 b.

[0154]FIG. 59 shows still another surface light source device 90 usingan L-shaped linear light source 54, having a quadrangular planar lightconducting plate with a light converging pattern 58 and a deflectionpattern provided in diagonal directions. (FIG. 59 does not show thedeflection pattern but it is diagonal, perpendicular to the lightconverging pattern.) An optical pattern 91 is formed on the lightincident side surfaces 53 opposite the linear light source 54. Lightbeams made incident into the light conducting plate 52 through the twolight incident side surfaces 53 are superposed inside the lightconducting plate 52 by the optical pattern 91, becoming a beam in thediagonal direction as shown in FIG. 59. In other words, although thelinear line source 54 is L-shaped, it is possible to align the emittedlight in one direction, as in the case of using a straight-shaped linearlight source.

[0155]FIG. 60 shows still another surface light source device 92 usingan L-shaped linear light source 54, having a quadrangular lightconducting plate 52 with a light converging pattern 58 and a deflectionpattern formed thereon in diagonal directions. A diffraction grating 93is attached to the light incident side surfaces 53 of the lightconducting plate opposite to the L-shaped linear light source 54 suchthat light entering the light conducting plate 52 through the lightincident side surfaces 53 and diffracted by the diffraction grading 93will be in the diagonal direction of the light conducting plate 52.

[0156]FIG. 61 shows still another surface light source device 94 usingan L-shaped linear light source 54, having a quadrangular lightconducting plate 52 with its light incident side surfaces 53 formed in atriangular wave-like design. Reflective mirrors 95 are attached toportions of the light incident side surfaces 53 facing in one directionand the other portions facing in the other direction are left asmutually parallel transparent surfaces 96. Thus, light emitted from thelight source 54 enters the light conducting plate 52, either directly orafter being reflected by one of the reflective mirrors 95, through oneof the transparent surfaces 96. Incident beams of light are thus alignedin the diagonal direction of the light conducting plate 52 and emittedout from the upper surface of the light conducting plate 52.

[0157]FIG. 62 shows another surface light source device 97 having aprism sheet 70 disposed opposite to the light emitting surface 56 of itslight conducting plate 52. Light is emitted from the light emittingsurface 56 nearly parallel to the light emitting surface 56 in theX-direction. The prism sheet 70 has a uniform pattern in the Y-directionwhich is perpendicular to the X-direction. With the prism sheet 70 thusdisposed, the emitted light in the X-direction is deflected to theZ-direction perpendicular to the device 97.

[0158] The bottom angle of the prism pattern 70 a (as shown in FIG. 63)of a commonly used prism sheet 70 is about 45 degrees to 50 degrees. Inorder that the light, after passing through the prism sheet 70, shouldtravel perpendicular to the sheet 70, the light should make an angle ofabout 60 degrees with the light emitting surface 56 of the lightconducting plate 52. If the light conducting plate 52 is designed suchthat light will be emitted from its light emitting surface 56 in such adirection, ranges r3 and r4, as defined above, become too large,adversely affecting directionality.

[0159] If the base angle of the prism pattern 70 a of the prism sheet 70is increased to about 70 degrees, as shown in FIG. 64, however, therequired angle of emission of light from the light emitting surface 56becomes about 30 degrees, and the light conducting plate 52 can bedesigned accordingly without requiring the ranges r3 and r4 to becometoo large and hence without adversely affecting directionality.

[0160] According to analyses, it can be ascertained that the base angleof the prism pattern 70 a should be greater than 60 degrees. In otherwords, if the direction of light entering the back surface of the prismsheet 70 is closer to being parallel than perpendicular thereto (such aswhen the angle of the emitted light from the light emitting surface 56is less than 45 degrees from the light emitting surface 56 as shown, forexample, in FIG. 64) and if it is desired to deflect the light to theperpendicular direction with respect to the prism sheet 70, Snell's lawrequires that the base angle of the prism pattern 70 a should be about60 degrees. If the index of refraction of the prism sheet 70 is smaller,Snell's law also requires that the base angle should be larger. Sincethe index of refraction of the prism sheet 70 cannot be much greaterthan 1.5, even if it is assumed to be 1.59, the required base angle ofthe prism patterns 70 a becomes 60 degrees.

[0161]FIG. 65 shows a prism sheet 70 having another structure which maybe used for a surface light source device embodying the invention. Asthe base angle of the prism pattern 70 a is made as large as 70 degrees,the top angle becomes about 40 degrees, and a prism sheet 70 with suchsharp tops are easily damaged. The prism sheet 70 of FIG. 65 hasflattened tops such that the sheet 70 will not be damaged easily. Onlythose portions of the pattern 70 a where light does not pass through areflattened, as shown by light paths in FIG. 65.

[0162]FIG. 66 shows another surface light source device 98 embodyingthis invention with a prism sheet 70 with a different design. Sincelight is emitted from the light conducting plate 52 in a direction awayfrom the linear light source 54, it is the slopes on the prism sheet 70on the side away from the light source that are used for deflecting thelight into the perpendicular direction. Accordingly, the base angle γ onthe side away from the light source is made larger than 45 degrees, orpreferably larger than 60 degrees, the base angle on the side closer tothe light source being made smaller.

[0163] With the prism pattern 70 a thus designed, even beams of lightemitted with a very small angle with the light emitting surface 56 canbe effectively deflected into a perpendicular direction because the baseangle γ of the effective portions of the deflecting surface issufficiently large. If this design is compared with the symmetric shapedrawn by dotted lines in FIG. 66, it can be easily understood that theprism sheet 70 shown in FIG. 66 is much less likely to be damaged.

[0164]FIG. 67 shows another surface light source device 99 having notonly a prism sheet 70 but also side surface reflecting plates 81. Notonly is light emitted from the light source but reflected light by thesereflecting plates 81 travels backwards towards the light source. Thus,use is made of a prism sheet having symmetric prism pattern 70 a suchthat reflected light can be emitted equally effectively from the lightemitting surface 56 of the light conducting plate 52.

[0165] The pitch of the prism sheet 70 should preferably so selectedthat it will not be an integral multiple of that of either theX-direction pattern of the light conducting plate 52 or that of thedeflection pattern, or the other way around, such that generation ofMoire fringers can be prevented.

[0166]FIG. 68 shows an image display apparatus 101 using a surface lightsource device 102 embodying this invention, comprising a surface lightsource device 102 which includes a linear light source 54, a reflector55, a light conducting plate 52, a reflecting plate 68 and a prism sheet70. A micro-lens array 103 comprising micro-lenses is disposed above thesurface light source device 102, and an LCD panel 104, structuredsimilarly to the LCD panel shown in FIG. 7, is disposed thereabove. Themicro-lenses of the array 103 are arranged so as to be in one-to-onecorrespondence with the openings 105 (shown in FIG. 69) between thepixels of the LCD panel 104.

[0167] As shown in FIG. 69, light beams emitted perpendicularly from thesurface light source device 102 can be converged to the openings 105 ofthe pixels by means of the micro-lens array 103 such that the blackmatrix 106 of the LCD panel 104 is prevented from screening the lightfrom the surface light source device 102. Since the surface light sourcedevice 102 is according to an embodiment of this invention, inparticular, the emitted light therefrom is unidirectionally aligned, andthe micro-lens array 103 can function effectively in focusing the lightat the openings 105 with high accuracy. In other words, light isutilized more efficiently and the front brightness of the image displayapparatus 101 is greater than that of a prior art image displayapparatus. Alternatively, however, use may be made of a surface lightsource device without a prism sheet, as shown in FIG. 70, by using adiffraction grating 107 to deflect the light emitted from the surfacelight source device 102 and then focusing the deflected light at theopenings 105.

[0168]FIG. 71 shows a color image display apparatus 108 using a surfacelight source device embodying this invention, using an LCD panel 104having color filters and comprising pixels of red (R), green (G) andblue (B). A diffraction grating 109 is provided between the surfacelight source device 102 and a micro-lens array 103. Beams of lightemitted perpendicularly from the surface light source device 102 arediffracted by the diffraction grating 109 into different directions,depending on the color, and the micro-lens array 103 serves to focusthem at corresponding pixels. The lowering of brightness due to thecolor filter can be effectively prevented since use is made of a surfacelight source device embodying this invention capable of aligning beamsof light to the diffraction grating 109.

[0169]FIG. 72 shows another color image display apparatus 110characterized as using a surface light source device 102 without a prismsheet. Because there is no prism sheet, light beams emitted from thesurface light source device 102 are made incidence onto the diffractiongrating 109 at a larger angle. The difference in diffraction angle bythe diffraction grating 109 due to difference in frequency increases asthe diffraction angle becomes larger. Thus, color separation becomeseasier if the emitted light from the surface light source device 102 isprojected diagonally onto the diffraction grating 109. This embodimentis further advantageous because the number of component is smaller.

[0170]FIG. 73 shows another image display apparatus 111 embodying thisinvention characterized as having a polarization separation element 112disposed between the surface light source device 102 and the LCD panel104 such that the portion of light emitted from the surface light sourcedevice 102 perpendicularly to the polarization separation element 112and polarized thereby in one direction (P-polarization) is allowed topass while the portion polarized in the other direction (S-polarization)is reflected. The reflected light is returned into the light conductingplate and emitted again therefrom. Thus, the image display apparatus 111according to this embodiment can prevent lowering of brightness due to apolarization plate. Since the surface light source device 102 accordingto this invention emits light unidirectionally, the effect of preventinglowering of brightness is further improved.

[0171] As explained above, image display apparatus using a surface lightsource device according to this invention can effectively preventlowering of brightness due to the black matrix, color filter orpolarization plate.

[0172]FIG. 74 shows an LC television set 113 using a surface lightsource device embodying this invention and containing an image displayapparatus 114 shown in FIG. 75 with an LCD panel 104 disposed above asurface light source device 102 having a prism sheet 70 and a diffusionplate 115 disposed above the LCD panel 104. The degree of diffusion ofthe diffusion plate 115 changes, depending on the direction, being highin the Y-direction and lower in the X-direction. The image displayapparatus 114 is disposed inside the LC television set 113 such that theY-axis is horizontal. The diffusion plate 115 need not be as describedin FIG. 75. Its average slope angles in sectional surfaces in theX-direction and the Y-direction may be different. The LC television set113 is so set that light is diffused horizontally by the diffusion plate115 in front but diffusion does not take place much in the up-downdirections. With the television set 113 thus structured, therefore, thedisplay is not difficult to see from diagonal directions. The televisionset 113 thus structured can be enjoyed by a large number of viewers.

[0173] Image display apparatus thus structured, because their displayscan be seen from different directions thanks to the diffusion plate 115,can be used not only in television sets but also in many other kinds ofapparatus such as video camera monitors and displays for personalcomputers. Depending on the purpose of use, a different diffusion platemay be used with ability to diffuse light both horizontally andvertically.

[0174] Although the diffusion plate 115 shown in FIG. 75 uses its uppersurface as its diffusion surface 115 a, it may be placed on the LCDpanel 104 with the diffusion surface 115 a facing downward, as shown inFIG. 76. If the diffusion surface 115 a faces downward, it becomescloser to the openings 105 between the pixels of the LCD panel 104, asshown in FIG. 77A, as compared to FIG. 77B. Thus, the image becomessharper and resolution improves.

[0175] As a further variation, a diffusion surface 115 a may be formeddirectly on the upper surface of the glass plate of the LCD panel 104,as shown in FIG. 78, instead of using a diffusion plate. This variationis advantageous in that the number of components is reduced and hencethe production cost can be also reduced. Moreover, a sharper image isobtainable since the diffusion surface 115 a becomes even closer to theopenings 105 of the pixels.

[0176] If a prior art LCD panel 116 is used, as shown in FIG. 79, thereis a black matrix 106 between two glass sheets, serving to prevent lightfrom reaching elements with poor on-off characteristics such as LC layerand TFT and to thereby maintain optical characteristics of the LCD panel116. According to this invention, since the surface light source device102 makes use of a micro-lens array 103 to focus light with accuracy atthe openings 105 of the pixels, the black matrix 106 of prior art LCDpanel 116 may be dispensed with, as shown in FIG. 80 by contrast.

[0177] If light happens to pass through outside the opening part of thepixels because the black matrix of the prior art LCD panel has beenremoved, a black matrix 117 may be provided as shown in FIG. 81 on theupper surface of the glass plate of the LCD panel 104. If a black matrixis thus provided externally to the LCD panel, rather than inside the LCDpanel 104, the production of the black matrix becomes much simplerbecause there is no need for patterning. A black matrix 117 may beprovided on the lower surface of the glass plate of the LCD panel 104,as shown in FIG. 82, especially when unidirectionally aligned light isdirectly passed to the LCD panel 104.

[0178]FIG. 83 shows an embodiment wherein another black matrix 118 isprovided to the diffusion plate 115 corresponding to the black matrix inthe LCD panel 104. With such a structure, the added black matrix 118serves to cover the parts where light from adjacent pixels is generated.Thus, pixel images can be made sharper. Such additional black matrix 118may be provided both on the upper and lower surfaces of the diffusionplate 115, as shown in FIG. 84. FIG. 85 shows still another embodimentwherein a black matrix 117 is provided on the glass panel of the LCDpanel 104 corresponding to the black matrix 106 formed between the glassplates of the LCD panel 104. With such a structure, light with largeangles of emission can be cut off by the added black matrix 117 suchthat the LCD 104 is capable of improving the directionality of emittedlight from the surface light source device 102. If use is made of adiffusion plate 115, pixel images can be made sharper for the samereason described above with reference to FIG. 83. Such an additionalblack matrix 117 may be provided on both the upper and lower surfaces ofthe LCD panel as shown in FIG. 86. If a black matrix is provided on twoor more planes, the directionality is improved and overlapping of pixelsis reduced.

[0179]FIGS. 87 and 88 show an automatic teller machine (ATM) 119 usingan image display apparatus 121 according to this invention. The screen120 of such an ATM 119 is preferably designed so as not to be visible topersons who may be standing next to the user. For this reason, the imagedisplay apparatus 121 is designed such that emitted light therefrom isfocused at the user, as shown in FIG. 88. A person standing next to theuser will be looking at the screen at least at an angle of about 30degrees, as shown in FIG. 89. If it is assumed that the distance Hbetween the screen 120 and the user is 55 cm and the separation Ebetween the user and the person of an ordinary size standing next to theuser is 30 - 35 cm, such a person standing next to the user will belooking at the screen 120 at an angle of over 30 degrees. Thus, if lightintensity for emitted light at angles outside the range of −30 to +30degrees is dropped to less than ⅕of the forwardly emitted light, thescreen will be practically invisible to anybody besides the user.

[0180]FIG. 90 shows a structure for focusing the light from the screenas shown in FIG. 88 by disposing a converging lens 122 above the LCDpanel 104. As shown in FIG. 91, however, the lens 122 may be disposedbelow the LCD panel 104. As shown in FIG. 92, furthermore, a Fresnellens 123 may be used as the converging lens such that the overallthickness can be reduced. With a structure as shown in FIGS. 90 - 92,the peak in brightness on the image display surface varies, focused atthe position of the user's head. With an image display apparatus 121according to this invention, since light is emitted in one direction, alens can be used together to easily vary the direction of emission. Suchimage display apparatus can be used not only in ATMs but in many otherkinds of apparatus such as game tables.

[0181]FIG. 93 shows an automobile provided with an automatic navigationsystem. Since such an automatic navigation system is usually set on thedash board between the driver and the front-seat passenger, the driverwill look at it from a diagonal direction. Prior art LC displays aredifficult to see from diagonal directions. In automatic navigationsystems according to the present invention, the display 125 is adjustedto be easily visible from diagonal directions.

[0182] Image display apparatus 126 for this purpose are shown in FIGS.94A, 94B and 94C. The apparatus 126 shown in FIG. 94A is characterizedin that a diffraction grating 127 is installed on the front surface ofthe LCD panel 104 such that light is emitted from the screen diagonallytowards the driver (not shown). The apparatus 126 shown in FIG. 94B hasa prism plate 128 disposed in front of the LCD panel 104 such that lightfrom the screen is emitted both to the right and to the left such thatit can be easily seen both by the driver and the front-seat passenger.The apparatus 126 shown in FIG. 94C does not use a prism sheet butcauses the light from the surface light source device to pass the LCDpanel diagonally.

[0183] LC display apparatus 131 are sometimes found inside a train, say,above the door 130 of a passenger car 129, as shown in FIG. 95.Passengers usually look at such a display diagonally from below. Sincethe LC display apparatus 131 is usually not installed so as to pointdownward, the display is sometimes very difficult to see. In such asituation, an optical sheet such as a diffraction grating or prism sheetmay be placed in front of the LCD panel 104 as shown in FIG. 94A suchthat light is not emitted upward so much but mostly in downwarddirections. Intensity of light going downward may be about 1.5 timesgreater than that going upward.

What is claimed is:
 1. A surface light source device comprising: a lightconducting plate having a light incident side surface, a light emittingsurface and an opposite surface which is opposite to said light emittingsurface; a light source disposed adjacent to said light incident sidesurface of said light conducting plate; a deflection pattern capable ofdeflecting light inside said light conducting plate gradually toward thedirection perpendicular to said light emitting surface; and a convergingpattern capable of causing light emitted from said light emittingsurface to be converged into a direction of a plane perpendicular tosaid light incident side surface and said light emitting surface; saiddeflecting pattern being on at least either of said light emittingsurface and said opposite surface, and said converging pattern being onat least either of said light emitting surface and said oppositesurface.
 2. The surface light source device of claim 1 furthercomprising a prism sheet disposed on said light emitting surface, saidprism sheet having a prism pattern which is uniform in one direction. 3.The surface light source device of claim 2 wherein said prism patternhas bottom angle which is larger than 60 degrees.
 4. A surface lightsource device comprising: a light conducting plate having a lightincident side surface, a light emitting surface and an opposite surfacewhich is opposite to said light emitting surface; a light sourcedisposed adjacent to said light incident side surface of said lightconducting plate; and at least one pattern on at least either of saidlight emitting surface and said opposite surface, the sum of averageslope angles of said light emitting surface and said opposite surface ona first sectional surface which is perpendicular to both said lightincident surface and said light emitting surface being greater than thesum of average slope angles of said light emitting surface and saidopposite surface on a second sectional surface which is parallel to saidlight incident side surface.
 5. The surface light source device of claim4 further comprising a prism sheet disposed on said light emittingsurface, said prism sheet having a prism pattern which is uniform in onedirection.
 6. The surface light source device of claim 5 wherein saidprism pattern has bottom angle which is larger than 60 degrees.
 7. Animage display apparatus comprising: a surface light source devicecapable of emitting parallel light from an extended light emittingsurface; a liquid crystal display panel having two transparent platesand being free of any black matrix between said two transparent platesfor screening light; and a micro-lens array disposed between saidsurface light source device and said light crystal display panel.
 8. Theimage display apparatus of claim 7 wherein said surface light sourcedevice comprises: a light conducting plate having a light incident sidesurface, a light emitting surface and an opposite surface which isopposite to said light emitting surface; a light source disposedadjacent to said light incident side surface of said light conductingplate; and at least one pattern on at least either of said lightemitting surface and said opposite surface, the sum of average slopeangles of said light emitting surface and said opposite surface on afirst sectional surface which is perpendicular to both said lightincident surface and said light emitting surface being greater than thesum of average slope angles of said light emitting surface and saidopposite surface on a second sectional surface which is parallel to saidlight incident side surface.